This proposal focuses on the mechanisms by which somatosensory neurons choose their peripheral arbor territories. Touch sensation in the face is accomplished by trigeminal sensory neurons. Each trigeminal neuron projects one peripheral axon that elaborates an intricately branched arbor in a discrete portion of the head. Together, the arbors of all trigeminal neurons blanket the entire epidermis with a network of sensory fibers. In previous work, we used the zebrafish trigeminal system as a model to investigate how trigeminal neurons coordinate with one another to create their even and comprehensive arbor arrangement. Using imaging and embryological approaches, we demonstrated that repulsive interactions between growing arbors limit their territories and ensure comprehensive innervation of the head epidermis. We showed with a behavioral experiment that removing these repulsive constraints disrupts the arbor territory map and impairs the ability of animals to locate stimuli in the environment. In humans, defects in the arrangement of trigeminal innervation territories that occur developmentally or as a result of injury can also lead to defects in touch sensation, including painful neuropathies. We propose here to take further advantage of the clarity, accessibility and molecular tractability of the zebrafish trigeminal system to address three related questions about how trigeminal neurons choose their peripheral territories during development and after injury, emphasizing the role of the repulsive interactions that shape arbor territories. First, we will explore whether subtypes of sensory neurons use independent repulsive systems to choose their territories and whether there is a correlation between neuronal shape and function. We will accomplish this by creating transgenes for independently monitoring the development and morphologies of each trigeminal sensory subtype in live embryos. Second, we will characterize the cellular and molecular strategies employed by trigeminal neurons during the re-establishment of territories after peripheral injury. For these experiments, we will employ a new method we have developed to inflict precise damage on trigeminal neurons with a laser. The clarity and accessibility of zebrafish trigeminal neurons allows us to follow the regeneration of single damaged trigeminal arbors at high-resolution in real time. Third, we will exploit the molecular and genomic tools available in zebrafish to identify the molecules that mediate mutual repulsion among trigeminal neurons. We will perform a comprehensive expression screen to determine which genes are the best candidates for participating in this process, and a loss-of-function approach to test their functions. With these studies, we hope not only to provide a description of the basic mechanisms used by neurons to coordinately choose their innervation territories, but also to shed light on the causes of trigeminal innervation defects that can contribute to facial neuropathies in humans. PUBLIC HEALTH RELEVANCE: Developmental defects in peripheral touch-sensing neurons, or damage to them later in life, can trigger painful neuropathies. Peripheral neuropathies are a common and heterogeneous collection of diseases, affecting some 20 million Americans, that can result in heightened and often persistent pain sensation, but very little is understood about them at the cellular and molecular level. The studies in this proposal aim at understanding the basic mechanisms that control the development and repair of touch-sensing neurons, which we believe will ultimately lead to better ways for treating patients who suffer from peripheral neuropathies.